1. Field of the Invention
This invention relates to binoculars having a focus adjusting device of the center focus type, and in particular to binoculars having a rotatable focus adjusting ring on an axis shaft for supporting a pair of right and left body tubes for rotation relative to each other.
2. Related Background Art
Numerous focus adjusting mechanisms have heretofore been devised in binoculars of the center focus type, and for example, the structure as shown is FIG. 7 of the accompanying drawings in known. In FIG. 7, washers 106a and 106b are sandwiched between the arm portions 101A, 101B and 102A, 102B of left and right bodytubes 101 and 102, and an axis shaft 103 is fitted in mating holes formed in the left and right arm portions 101A and 102A. The left and right bodytubes 101 and 102 are rotatable about the axis shaft 103 due to said fitting, and by fastening an axis shaft fastening screw 104, a desired smooth rotational coupling is provided through the washers 106a and 106b sandwiched between the arm portions 101A, 101B and 102A, 102B of the left and right bodytubes. A central axis pestle 107 is axially slidably fitted in the bore of the axis shaft 103, and the head of a small screw 110 studded in the central axis pestle 107 is engaged with an axially elongated slot 103A formed in the axis shaft 103 and slides in the lengthwise direction of the slot 103A to enable the rectilinear movement of the central axis pestle 107.
A focus adjusting ring 111 is fitted around the axis shaft 103, and the end portion of a small screw 109 secured to the focus adjusting ring 111 fits in a circumferential groove 103b formed in the outer periphery of the axis shaft 103 to enable only the rotational movement of the focus adjusting ring 111 and limit the axial sliding movement thereof. A central axis ring 108 is provided in the space between the axis shaft 103 and the focus adjusting ring 111, and a lead thread formed on the outer periphery thereof is threadably engaged with a helicoid thread formed on the focus adjusting ring 111 and the bore thereof slidably fits to the axis shaft 103. The small screw 110 studded in the central axis pestle 107 has its head fitted in a hole formed in the central axis ring 108.
Description will hereinafter be made of the focus adjusting operation in the prior-art binoculars constructed as described above. When the focus adjusting ring 111 is rotated, the central axis ring 108 threadably engaged with the helicoid thread formed on the bore of the focus adjusting ring 111 is moved in accordance with the lead thread thereof. With the movement of the central axis ring 108, the small screw 110 studded in the central axis pestle 107 fitted to the axis shaft 103 rectilinearly moves (axially moves) along the slot 103A formed in the axis shaft 103. Accordingly, the central axis pestle 107 is axially moved to axially move an eyepiece tube 112 through an eyepiece bridge 113 supported by the central axis pestle 107 and enable focus adjustment.
The prior-art binoculars of the center focus type are divided broadly into ones provided with a focus adjusting ring between the upper and lower arm portions as described above and ones provided with a focus adjusting ring between the eyepiece bridge 113 and the arm 102B. There have been no binoculars of either focus adjusting type in which focus lock is possible at a desired position.
In the prior-art focus adjusting device as described above, observation is effected at a desired observation distance with the focus adjusting ring rotated as appropriate. It is quite common to conduct observation of an object continuously or intermittently at the same observation distance over a long time.
For this purpose, the binoculars should be held so that the focus adjusting ring does not rotate. It is surprisingly difficult, however, to hold the binoculars in their in-focus state when observation is discontinued or when the binoculars are handed over to another observer. Also, because the focus adjusting ring is designed to be readily rotated, there has been the problem that even if substantial care is taken, the ring is rotated with a slight force and so deviates from the initial in-focus position. Thus, it becomes necessary to perform the focusing operation again to match the in-focus distance with the object of observation.